How Should an MMO Cash Shop Work?

There was once a time where the very notion that an mmorpg had a cash shop would have sent gamers into convulsions. The prominent thought during that time was that an mmo cash shop was just an exercise in ripping off the players and a mark of the dreaded free-to-play game. While such a time existed just a few short years ago, it feels like almost a century to older players. The f2p model has emerged triumphant over subscription-based games, and every f2p game needs its own mmo cash shop to generate revenue. Most gamers happily accept this fact, but there are a few that still grumble about pay-to-win every chance they get. Still, there are good ways to run a cash shop for an online game and there are bad ways. How should an mmo cash shop work?